In the semiconductor production industry, various processing steps are used to fabricate integrated circuits (ICs) on a semiconductor wafer. These steps include the deposition of layers of different materials including metallization layers, passivation layers and insulation layers on the wafer substrate, as well as photoresist stripping and sidewall passivation polymer layer removal. In modern memory devices, for example, multiple layers of metal conductors are required for providing a multi-layer metal interconnection structure in defining a circuit on the wafer. Typically, multiple alternating layers of electrically conductive and insulative materials are sequentially deposited on the wafer substrate, and conductive layers at different levels on the wafer may be electrically connected to each other by etching vias, or openings, in the insulative layers and filling the vias using aluminum, tungsten or other metal to establish electrical connection between the conductive layers. Chemical vapor deposition (CVD) processes are widely used to form layers of materials on a semiconductor wafer. Other processing steps in the fabrication of the circuits include formation of a photoresist or other mask such as titanium oxide or silicon oxide, in the form of the desired metal interconnection pattern, using standard lithographic techniques; subjecting the wafer substrate to a dry etching process to remove the conducting layer from the areas not covered by the mask, thereby leaving the metal layer in the form of the masked pattern; removing the mask layer using reactive plasma and chlorine gas, thereby exposing the top surface of the metal interconnect layer; cooling and drying the wafer substrate by applying water and nitrogen gas to the wafer substrate; and removing or stripping polymer residues from the wafer substrate.
Throughout the IC fabrication process, the work-in-progress (WIP) wafer substrates must be sequentially transferred among multiple processing stations. The wafer-transfer operation must be carried out under stringent cleanliness conditions and at various pressures. Accordingly, numerous mechanical arrangements have been devised to facilitate transferring wafers from one processing chamber to another or from a transport vehicle such as an automatic guided vehicle (AGV) or an overhead hoist transport (OHT) vehicle.
Typically, multiple semiconductor wafers are loaded into a wafer cassette such that the wafers can be transported and processed in multiple-wafer lots. A cassette loaded with wafers is inserted into an input/output (I/O) chamber, or “load lock” chamber, where a desired gas pressure and atmosphere can be established. The wafers are fed one-by-one to or from their respective cassettes into or out of the load lock chamber. It is desirable from the standpoint of efficiency in handling of the wafers that the loadlock chamber be located in close proximity to a number of processing chambers to permit more than one wafer to be processed nearby and at the same time. To this end, two or more chambers are arranged at locations on the periphery of a transfer chamber which is hermetically sealable and communicates with both the loadlock chamber and the processing chambers. Located within the transfer chamber is an automatically controlled wafer handling mechanism, or robot, which takes wafers from the loadlock chamber and then transfers each wafer into a selected processing chamber. After processing in one chamber, a wafer is withdrawn by the robot and inserted into the next processing chamber in the processing sequence. Ultimately, the wafer is returned to the load lock chamber and into a cassette for transport to the next processing station.
WIP semiconductor wafers are fragile and easily chipped, cracked or scratched. Therefore, the wafers must be handled with great care during processing and transfer. Wafer transfer robots are designed to handle wafers securely and yet without scratching the surface or cracking or chipping an edge of the wafers. The robot transfers each wafer in a smooth motion without vibration or sudden stops or jerks. Vibration of the robot blade could cause abrasion between the blade holding the wafer and the surface of the wafer. This could result in the formation of potential circuit-contaminating particles on the wafer or other wafers to be handled by the robot. As a result, the design of a robot requires careful measures to ensure that the movable parts of the robot operate smoothly without lost motion or play, with the requisite gentleness in holding a wafer, and yet be capable of moving the wafer quickly, smoothly and accurately between two locations.
Referring to FIGS. 1–3, a typical conventional. wafer robot-actuating system 10 includes a control board 12 which is connected to a step motor driver 14 that drives a step motor 16. A lead screw 20 is coupled to the drive shaft 18 of the step motor 16. A blade collar 22, to which is attached a robot blade 24, threadibly engages the lead screw 20. The control board 12 sends a command signal 32 to the step motor driver 14, which actuates the step motor 16 to rotate the motor drive shaft 18 and lead screw 20. Accordingly, depending on the direction of rotation of the lead screw 20 as determined by the control board 12, the robot blade 24 moves in the Z-axis direction toward or away from a wafer 46 preparatory to either picking up the wafer 46 or removing the wafer 46 from a wafer cassette 44, respectively. An encoder disk 28 is provided on an encoder shaft 26 drivingly engaged by the step motor 16. An encoder 30 is provided in magnetic proximity to the encoder disk 28 and provides a feedback signal 34 to the control board 12 indicating the number of revolutions of the encoder disk 28, and thus, the Z-axis position of the robot blade 24.
As shown in FIG. 2, the robot blade 24 typically extends from a blade seat 36 and is fitted with a rubber or plastic gasket 38 having a vacuum opening 40. Vacuum pressure is applied to the vacuum opening 40 through a vacuum tube 42 as the gasket 38 contacts one of the wafers 46 in the cassette 44. The wafer 46 is thus held against the gasket 38 by vacuum pressure as the robot blade 24 lifts the wafer 46 from the cassette 44, which may be submerged in DI water 48 in some chemical mechanical polishing (CMP) applications, for example.
One of the problems commonly encountered in operation of the robot actuating system 10 is that the robot blade 24 has a tendency to strike the edge of one or more of the wafers 46 in the cassette 44 as the blade 24 is lowered into position to lift the wafer 46 from the cassette 44. This causes cracking, chipping and/or abrasion of the wafer 46, resulting in damage to the wafer or the generation of potential circuit-contaminating particles. Conventional blade/wafer collision prevention systems utilize an optical signal which indicates to the control board that the blade is in danger of striking and damaging the wafer or wafers. However, in cases in which the robot blade 24 must be submerged into DI water 48 to pick up the wafer or wafers 46, the optical signal is interrupted or distorted by ripples in the DI water 48. This interferes with the ability of the wafer robot to timely intervene and prevent harmful striking of the robot blade with the wafer. Accordingly, a wafer protection system is needed for preventing a robot blade from striking or pushing against a wafer in a cassette and which is adaptable to wafer transfer robots used to remove wafers from DI water.
An object of the present invention is to provide a new and improved system for preventing damage to a wafer resulting from striking of a robot blade with the wafer.
Another object of the present invention is to provide a wafer protection system which may be adapted to any type of semiconductor processing system.
Yet another object of the present invention is to provide a wafer protection system which senses improper positioning of a wafer transfer robot blade with respect to a wafer and terminates further movement of the blade to prevent blade-induced damage to the wafer.
Still another embodiment of the present invention is to provide a wafer protection system which utilizes electrical contact between a foil conductor and a wafer robot blade to terminate continued movement of the blade and prevent cracking, chipping or abrasion of the wafer.
A still further object of the present invention is to provide a wafer protection system which is applicable to wafer transfer robot blades that are immersed in water or other liquid during the wafer transfer operation.
Another object of the present invention is to provide a wafer protection system which is simple in design and operation.